A general principle for operating hybrid vehicles consists in operating either the electric motor, or the engine, or else both at the same time, depending on the model.
One of the main features is as follows:                during stationary stages (when the vehicle is not moving), both the engine and the motor are stopped;        on starting, it is the electric motor that serves to put the car into motion, up to higher speeds (25 kilometers per hour (km/h) or 30 km/h);        when higher speeds are reached, the engine takes over;        in the event of high acceleration, both the engine and the motor are operated together, thereby making acceleration possible that is equivalent to that of an engine of the same power, or even greater; and        during a deceleration and braking stage, kinetic energy is used to recharge the batteries (it should be observed that this feature is not available on all hybrid vehicles available on the market at present).        
It can be seen that the engine does not operate continuously, and that under such circumstances, it is not possible to perform stages of purging the canister (an active carbon filter that avoids dumping fuel vapor into the atmosphere), because during such stages, optionally pre-heated air is caused to flow through the canister in order to regenerate it (i.e. in order to desorb the fuel vapor that has been adsorbed therein), with that air then being admitted into the engine where it is burnt.
Under such circumstances, in order to avoid loading the canister pointlessly, communication between the tank and the canister is generally interrupted by default; as a result, the fuel tanks of such vehicles are generally put under pressure (typically a pressure of the order of 300 millibars (mbar) to 400 mbar), with this generally being done by a functional element that is situated after the ventilation valves, often referred to as a fuel tank isolation valve (FTIV) and that prevents the tank being ventilated (degassing) other than during filling situations. This element generally comprises two safety valves (calibrated to the low and high maximum pressures at which the tank can be used) together with control means, generally electrical control means, in order to be able to bring the tank to atmospheric pressure before filling.
Compared with their metal counterparts, tanks (or other portions of the fuel system) that are made of plastics material present certain advantages in terms of weight and ease of preparation. Nevertheless, they can give rise to problems on being subjected to positive and/or negative pressure exceeding the above-mentioned “threshold” values, in the event of being subjected thereto for a prolonged duration. It can thus happen that there is cracking under stress, which can be made worse by exposure to the fuel, temperature, . . . .
It is possible to distinguish several types of cracking:                cracking of the skin of the tank;        cracking of reinforcing elements added inside the tank to limit deformation thereof during periods in which it is subjected to internal positive or negative pressure; and        cracking on some other element of the system.        